Gas-energized, gravity flow wall furnaces have been widely used for many years to provide heat for one or two rooms, typically in structures not having central heating. These furnaces are usually partially recessed into a wall in the space between two studdings in a conventional stud wall. Such space is normally only about 143/8 inches wide. The furnace must also be very shallow. Since a conventional stud only provides a space of about 31/2 inches in depth, the furnace usually extends only another 6-7 inches into the room. As a result of these dimensional constraints, and because of costs, many such wall furnaces do not have a fan and rely only on gravity for the flow of room air and combustion products. That is, the cool room air sinks and the warm room air and the hot combustion gases rise. Some furnaces, which rely primarily on gravity for the flow of the room air across the surface of the exchanger, also have a blower to redirect the heated air which is flowing upward due to gravity.
Current wall furnaces typically include a thin, flat, wide heat exchanger extending vertically in the wall, with the edges of the exchanger being positioned adjacent to, but spaced from, the studs in the wall. Air is drawn into the combustion chamber of the furnace and just past the burner port level. It is drawn into the flame by a natural entrainment or aspirating action. The flame jet, surrounded by a mixture of combustion products and secondary air, forms a recirculatory vortex which is drawn up along the boundary of the flame, and then down along the combustion chamber wall. The cause of this recirculation is a combination of the effects of initial vertical velocity of the gas-air mixture issuing from the burner, its expansion due to its increase in temperature as it burns and the buoyancy of the heated mixture relative to surrounding cooler gases. In this recirculation zone, the temperature of the combustion chamber walls is increasing to a certain maximum. A maximum wall temperature is reached at approximately the interface between the recirculation zone and a parallel flow zone. From this point on, the system functions substantially as a parallel flow heat exchanger and the combustion gases are ducted upwardly through the primary exchanger section.
The room air to be heated flows through a grill, forming the wall of the furnace facing the room. Cool room air enters the furnace near the lower end of the heat exchanger, is heated from the exterior of the heat exchanger, and flows upwardly due to a decrease in density caused by heating, and exits back into the room at the upper end of the heat exchanger. With this simple arrangement, fairly effective heat transfer is obtained.
Typically, the highest thermal efficiency provided by such gravity flow wall furnaces has been about 70%. This roughly means that the combustion process itself is fairly complete, and that at steady state about 70% of the heat from the combustion gases is transferred into the room. An alternative measure of heat transfer efficiency is the annual fuel utilization efficiency ("AFUE"). Typical gravity flow wall furnaces attain an AFUE of approximately 63-64%.
U.S. Pat. No. 4,784,110 discloses an improvement over the above gravity flow wall furnaces in which two secondary heat exchangers are positioned adjacent the main heat exchanger and combustion chamber combination. Thus, increased flow path, and therefore heat exchange area, is obtained by defining a tortuous path for the combustion gases. The secondary heat exchangers comprise sections or legs of the path extending side by side between the studdings in a conventional stud wall. The main combustion chamber is in the center of the two secondary exchangers. The disclosed improvement obtains an overall greater heat exchange area, but decreases the surface area directly facing the room to be heated. Further, all three exchanger sections are of identical length. Thus, the flue gases in the secondary exchangers are reheated to some extent by heat transfer from the adjacent combustion chamber. Although this design represents a significant improvement over standard wall furnaces, further improvements are possible.
Typical heat exchangers represent a trade-off between gas flow velocity and heat exchange surface area to obtain the maximum heat transfer. The combustion gases must flow upwardly through the heat exchanger with sufficient velocity to ensure that adequate air is drawn into the burner to provide sufficient oxygen and to produce a continued flow. At the same time, it is desirable that the velocity of the combustion gases be sufficiently slow and the surface area of the exchanger be sufficiently large to maximize the heat transfer from the heat exchanger.
The combustion gases of the typical furnace exits the structure through a flue spaced within the structure walls. In order to prevent overheating of the structure walls and avoid any potential fire hazard, the flue gases must be below a certain maximum temperature. The furnace must also minimize any down draft from the flue, which would adversely affect the burner operation. To this end, typical gravity flow wall furnaces include draft hoods at the top of the exchangers. The draft hoods collect flue gases and allow them to expand. The expansion in the draft hood results in some decrease in temperature of the flue gases, thereby decreasing the velocity of the flue gases and increasing their pressure. Thus, the risk of a down draft from the exterior is minimized.
The draft hood, however, needs to be vented to provide further cooling and to provide a secondary outlet for flue gases in the event the flue is blocked by an obstruction. Current wall furnaces locate the draft hood vents on the front of the draft hood. This vent location reduces the efficiency of the furnace by allowing some already heated room air to enter the vent and be discharged through the flue without heating the room.
In the operation of gravity flow furnaces, absent a deflector, the heated room air will generally rise to the top of the room. Even with deflectors, there is some degree of stratification of the hot and cool air. Accordingly, some gravity flow wall furnaces use blowers to direct the heated air out into the room. The blower is usually located at the top of the unit, just above the draft hood. The blowers are normally of a lower power variety than, for example, the blowers of the forced air type wall furnace, since they merely redirect air which is already circulating over the exchangers by the operation of gravity.
The blowers necessarily must be placed at the front of the exchanger in order to direct the heated air out into the room. This placement of the blower interferes with the operation of the vents on the draft hood, i.e., the blower is directing air out into the room and the vent is drawing air from the room. Thus, the increase in velocity created by the blower decreases the pressure in front of the vent, thereby decreasing the amount of air drawn in through the vents. These redirecting blowers are to be distinguished from the fans of forced air type furnaces, which are generally much more powerful and usually force air downward over the exchangers and out of the bottom of the unit into the room.
In recent years, a further requirement involving conservation of energy has been governmentally mandated. This was primarily imposed with respect to forced air, central, gas heating systems, but the regulations have been extended to be applicable as well to wall furnaces. The 70% thermal efficiency ratings of current commercial wall furnaces barely satisfy this requirement. The Department of Energy was to determine by January 1992 whether efficiency requirements were to be increased. Although the Department of Energy has not yet acted, it is anticipated that more stringent requirements may be proposed. It is expected that current wall furnaces will not meet a more stringent efficiency requirement, since most barely meet current requirements.
Thus, a need exists for an improved heat exchanger for a wall furnace that will improve efficiency, conserve energy and meet the various standards anticipated. The standards must be met based on a gravity flow system, i.e., without the use of a blower to circulate room air or a fan to induce draft for the combustion process. Of course, blowers can be used to further increase the efficiency of the furnace. Further, any such improvement must also be practical and inexpensive in order to be competitive from a marketing standpoint.